1. Field of the Invention
The invention relates to slotted-waveguide microwave antennas generally.
2. Description of the Prior Art
In the prior art, waveguide antennas employing slots for radiating elements are of themselves well known. Basic waveguides have been manufactured as rectangular pipes or tubes dimensioned in accordance with the known criteria for the frequency band of interest. Alloys of copper have been extensively used as a waveguide material as has extruded aluminum. In order to manufacture a waveguide, slot-radiator antenna from such a waveguide, the slots must be individually and accurately machined into a wall of the guide. In the form of slotted-waveguide antenna illustrated herewith, a dual-slot configuration is contemplated. It is to be noted that alternate slot pairs are often differently oriented along the waveguide face, that fact further complicating the slotting process according to prior art methods and increasing the cost in terms of skilled labor. The dual-slot, radiating, waveguide antenna per se is known in the prior art and is described in U.S. Pat. No. 3,740,751, entitled "Wideband Dual-Slot Waveguide Array," as it might be manufactured according to known prior art techniques.
It is also to be understood that slotted-waveguide antennas are frequently used in planar arrays. It is not unusual for each length of waveguide in such an array to require as many as 164 slots in each of 50 or 100 stacked waveguides. Accordingly, it will be readily perceived that machining expense is very significant in the manufacture of a large planar array for a radar system.
It is well known that slotted waveguide antennas and arrays require environmental protection, that is the array slots must be covered with a dielectric material or radome which acts as a radio frequency window while excluding dust, moisture and other undesired foreign matter. In the prior art extruded metal waveguide arrangement this must be accomplished as a separate operation and structure. Such expedients as hand-applied dielectric tape or individually formed fiberglass radomes for each of the individual slotted waveguides have been employed for that purpose.
Still further, the fact that metallic waveguide, especially aluminum waveguide, even though finished with protective coatings, is ultimately subject to attack from various combinations of environmental conditions and for proper operation periodic refurbishment is required.
Yet again, the prior art extruded aluminum waveguide array is relatively heavy and when the planar array is mounted for rotation (about the vertical axis) a large mechanical drive and pedestal support is required when such arrays are mounted so as to avoid obstacles. This frequently means mounting at substantial elevation, for example at the top of a mast of a naval vessel. In such a situation it will readily be understood that reduction in weight is a matter of great importance.
Yet another consequence of the use of aluminum waveguide, which is extensively employed in slotted-radiator planar arrays currently, is the introduction of distortions to the array radiation pattern caused by the relatively high coefficient of thermal expansion of the metal. Very often, the environmentally imposed temperature differentials impose serious limitations on the performance of large planar arrays exposed to the environment. Even under what may appear to be a relatively uniform temperature condition, differences of several degrees over a large array are not uncommon due to micro-climatic affects of thermal eddies from ground or water and random cooling affects of wind gust. Radiant heat from the sun can also produce relatively severe temperature differentials in the antenna array itself and its backing and support structure. In addition to the foregoing, the prior art all-metal waveguide generally contains no integral provisions for mounting. Attachment methods currently employed include such expedients as clamping the waveguide elements to the backstructure by means of machined aluminum bars notched to accommodate the waveguide, a substantial number of such clamps being required across the face of a completed array each bolted to a similarly machined aluminum member which is a part of the backstructure. Due to the fact that the front clamps infringe into the radiating aperture of the array they cause distortion of the radiation pattern of the overall array in addition to adding significantly to overall weight.
The manner in which the present invention deals with the disadvantages of the prior art as aforementioned will be evident as this description proceeds.